Wire bonding apparatus

ABSTRACT

A wire bonding method for joining a metal wire with a bonding pad disposed on a semiconductor element by using a load and supersonic wave vibration, comprising: during interval of time from contact of the metal wire with the bonding pad to application of the supersonic wave vibration, continuously applying a first bonding load and a second bonding load which is lower than the first bonding load; and after application of the supersonic wave vibration, continuously applying a third bonding load of a size of about 50% of the load of the second bonding load and a fourth bonding load which is lower than the first bonding load and higher than the third bonding load. The reliability of the fine wire bonding joint is improved remarkably, whereby a high quality semiconductor device cna be produced at a low cost.

This application is a division of application Ser. No. 08/835,802 filedon Apr. 16, 1997. Now U.S. Pat. No. 5,838,071.

BACKGROUND OF THE INVENTION

The present invention relates to a wire bonding method for improving thereliability of the fine wire bonding joint, a wire bonding apparatus,and a semiconductor device formed by using the wire bonding method andapparatus.

Heretofore, in the assembling process of semiconductor devices, therehas been extensively used a method of using both thermal pressurebonding and supersonic wave vibration as a method of bonding a metalwire to the bonding pad on a semiconductor device. To meet the trend ofreduction in size and pitch of the bonding pad in recent years,establishment of the wire bonding method for obtaining the bonding inhigher reliability is necessitated. FIG. 24 shows a relationship betweenthe load and application timing of supersonic wave vibration and thebonding time by the wire bonding method shown for example in JapaneseUnexamined Patent Publication No. 279040/1992. According to theembodiment, there is adopted a multi-stage loading by employing a lowload A between the high load and the low load B, so as to make the lowload A lighter than the low load B. The application of the supersonicwave vibration is commenced at the time of the high load.

As described above, according to the conventional wire bonding method,the bonding load amount for duration from the contact of the metal wirewith the bonding pad to the time of application of the supersonic wavevibration is constant. For this reason, there have been problems that,when the load amount at such time is set to a high amount to such adegree that nucleus of bond is formed, the deformation of metal wirebecomes excessively large, while on the other hand, when the deformationamount is set to a low level to such a degree that the deformationamount can be suppressed, the nucleus of bond cannot be sufficientlyformed. Furthermore, since the supersonic wave vibration is additionallyapplied during the application of load, the tendency for deformation ofthe metal wire to become excessively large is further accelerated, sothat there has been a problem that formation of a fine wire bondingjoint becomes impossible.

The present invention has been developed to solve the problems asdescribed above, and an object of the present invention is to provide awire bonding method and apparatus in which the nucleus of bond issufficiently formed at the joint between the metal wire and bonding pad,and deformation amount of the metal wire can be set to a moderate level,so that the reliability of fine wire bonding joint is improved, and toobtain a high quality semiconductor device having highly reliable bondby using the above method and apparatus.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided asemiconductor device comprising a semiconductor element, a lead frameconnected to the semiconductor element with a die bond material, and ametal wire for electrically connecting the semiconductor element and thelead frame, wherein a joint between the bonding pad disposed on thesemiconductor element and the metal wire comprises a plurality of islandshaped joints and a band-like joint surrounding the whole island shapedjoints.

Further, the lead frame includes Cu as a main component, and the diebond material includes resin as a main component.

Moreover, the metal wire includes Au as a main component, and thebonding pad includes Al as a main component.

In accordance with the present invention, there is also provided a wirebonding method for joining a metal wire with a bonding pad disposed on asemiconductor element by using a load and supersonic wave vibration,comprising: during interval of time from contact of the metal wire withthe bonding pad to application of the supersonic wave vibration,continuously applying a first bonding load and a second bonding loadwhich is lower than the first bonding load; and after application of thesupersonic wave vibration, continuously applying a third bonding load ofa size of about 50% of the load of the second bonding load and a fourthbonding load which is lower than the first bonding load and higher thanthe third bonding load.

Also, the metal wire has a metal ball at a tip portion of the metal wireto be connected to the bonding pad.

The bonding load amount divided by the sectional area prior todeformation of the metal ball is made to 40-60 mgf/μm² at the time ofthe first bonding load, 10-20 mgf/μm² at the time of the second bondingload, 4-10 mgf/μm² at the time of the third bonding load, and 10-20mgf/μu² at the time of the fourth bonding load.

Furthermore, the time of application of the first bonding load is notmore than 3 ms, the time of application of the third bonding load is 5to 15 ms, and the time of application of the fourth bonding load is 1 to5 ms.

In accordance with the present invention, there is further provided awire bonding apparatus for joining a metal wire with a bonding paddisposed on a semiconductor element by using a load and supersonic wavevibration, comprising: a stage to place a semiconductor device includingthe semiconductor element; a bonding head for positioning the metal wireon the bonding pad with holding the metal wire, and applying a load andsupersonic wave vibration; a mechanism to monitor change with time ofthe load amount of the bonding head; and a load control mechanism and asupersonic wave amplitude control mechanism having a conversion functionor conversion table to show correlation between the change with time ofthe load amount and the strength and deformation of the joint, and onreceipt of result from the monitor mechanism, calculating a subsequentload amount and amplitude of the supersonic wave vibration to controlthe bonding head.

BRIEF EXPLANATION OF THE DRAWINGS

FIGS. 1(a) and 1(b) show a constitution of a semiconductor device forillustrating a wire bonding method of Embodiment 1 of the presentinvention, wherein FIG. 1(a) is a sectional view and FIG. 1(b) is apartial sectional view of the joint portion;

FIG. 2 is a view to show relationship between load and timing forapplication of supersonic wave vibration, and joint time according tothe wire bonding method of Embodiment 1 of the present invention;

FIG. 3 is a view to show the joint condition after application of thethird load at a temperature of 210° C. in Embodiment 1;

FIG. 4 is a view to show the joint condition after application of thethird load at a temperature of 230° C. in Embodiment 1;

FIG. 5 is a view to show the joint condition after application of thethird load at a temperature of 250° C. in Embodiment 1;

FIG. 6 is a view to show the joint condition after application of thethird load at a temperature of 280° C. in Embodiment 1;

FIG. 7 is a view to show the joint condition after application of thethird load at a temperature of 210° C. in the case where amplitude ofthe supersonic wave is made smaller by 40% in Embodiment 1;

FIG. 8 is a view to show the joint condition after application of thethird load at a temperature of 280° C. in the case where amplitude ofthe supersonic wave is made smaller by 40% in Embodiment 1;

FIG. 9 is a view to show the joint condition after application of thethird load at a temperature of 210° C. in the case where applicationtime of the third load is made to 3 ms in Embodiment 1;

FIG. 10 is a view to show the joint condition after application of thethird load at a temperature of 210° C. in the case where applicationtime of the third load is made to 5 ms in Embodiment 1;

FIG. 11 is a view to show the joint condition after application of thethird load at a temperature of 210° C. in the case where applicationtime of the third load is made to 10 ms in Embodiment 1;

FIG. 12 is a view to show the joint condition after application of thethird load at a temperature of 210° C. in the case where applicationtime of the third load is made to 30 ms in Embodiment 1;

FIG. 13 is a view to show the joint condition after application of thethird load at a temperature of 210° C. in the case where applicationtime of the third load is made to 50 ms in Embodiment 1;

FIG. 14 is a view to show the joint condition after application of thethird load in the case where the first load is reduced to 100 gf and thesecond load to 30 gf in Embodiment 1;

FIG. 15 is a view to show the joint condition after application of thethird load in the case where the third load is increased to 30 gf inEmbodiment 1;

FIG. 16 is a view to show the joint condition in the case where thesample of 210° C. in FIG. 3 is preserved at a temperature of 150° C. for15 hours;

FIG. 17 is a view to show the joint condition in the case where thesample of 280° C. in FIG. 3 is preserved at a temperature of 150° C. for15 hours;

FIG. 18 is a view to show the joint condition after application of thefourth load by the wire bonding method of Embodiment 1 of the presentinvention;

FIG. 19 is a view showing one joint condition of FIG. 18;

FIG. 20 is a view to show the joint condition by a wire bonding methodof Embodiment 2 of the present invention;

FIG. 21 is a view to show the joint condition by the wire bonding methodof Embodiment 2 of the present invention;

FIG. 22 is a view to show relationship between time for application ofthe load and forming condition of inter-metal compound at the joint by awire bonding method of Embodiment 3 of the present invention;

FIG. 23 is a view to show the constitution of a wire bonding apparatusof Embodiment 4 of the present invention; and

FIG. 24 is a view to show relationship between load and timing forapplication of supersonic wave vibration, and joint time by theconventional wire bonding method.

DETAILED DESCRIPTION Embodiment 1

Hereinafter, the first embodiment of the present invention will beexplained with reference to the drawings. FIG. 1(a) is a cross-sectionalview to show the constitution of the semiconductor device forillustrating the wire bonding method according to the first embodimentof the present invention, and FIG. 1(b) is a partial cross-sectionalview showing the joint portion. Further, FIG. 2 is a view to showrelationship between load and timing for application of supersonic wavevibration, and joint time in the wire bonding method of this embodiment.In the drawings, numeral 1 shows a semiconductor element, 2 a bondingpad disposed on the semiconductor element 1, 3a a metal wire, 3b a metalball formed at the tip of the metal wire 3a, 4 a lead frame, 5 a diebond material for connecting the semiconductor element 1 and the leadframe 4, and 8 a capillary, respectively.

In this embodiment, the semiconductor element 1 is mechanically andelectrically connected to the lead frame 4 comprising mainly Cu by usinga die bond material 5 which comprises mainly polyimide and epoxy resin,having phenol as a curing agent, and filled with silver powder. Thefollowing explanation is given on the case where a metal wire 3acomprising mainly Au of 30 μm diameter and having a metal ball 3b ofabout 55 μm in diameter, made by melting and solidifying the tip of themetal wire 3a is joined to a bonding pad 2 of 80 μm in one sidecomprising mainly Al and formed on the semiconductor element 1, by usingthe load and supersonic wave energy of about 60 kHz supplied through thecapillary 8 and heat supplied from the lower face of the semiconductorelement 1. FIGS. 3 to 19 are views to show forming conditions ofinter-metal compounds at the joint portion between the metal ball 3b andthe bonding pad 2. In FIG. 19, numeral 6a shows a joint nucleus servingas a starting point of joint, 6b an island-shaped joint, and 7 aband-like joint formed in a manner to surround the whole joint 6b.Further, in each of FIGS. 3 to 18 and FIGS. 20 to 21, the upper figure(a), the middle left figure (b), the middle right figure (c), and thelower figure (d) show respectively a typical joint portion at the upperside, a typical joint portion at the middle left side, a typical jointportion at the middle right side, and a typical joint portion at thelower side of a rectangular semiconductor device. In FIGS. 3 to 18 and20 to 21, whitish or gray portions at about a center of the figures showisland-shaped joints (refer to FIG. 19).

Firstly, after contact of the metal ball 3b with the bonding pad 2, thefirst bonding load amount is raised in a sharp gradient to about 120 gfto produce mutually abrupt plastic flow on a joint interface between themetal wire 3a and the bonding pad 2. The sharp gradient rise of the loadcan be realized by the high speed descending motion of the capillary 8.By this plastic flow it is possible to exclude locally oxide film andthe like on the material surface by the load only, without supplyingsliding energy at the interface of contact, and to form evenly in thecontact interface a joint nucleus 6a which becomes the starting point ofthe joint. In such a case, the surface of the bonding pad 2 might beprovided with a projection of about several μm so as to facilitateformation of the joint nucleus 6a. For forming the projection, theremight be employed a method of growing an A1 single crystal projection byproviding for example a bonding pad 2 with heat treatment.

Thereafter, the second bonding load amount until application ofsupersonic wave vibration is controlled to a low amount of about 40 gfat which the vibration of the horn holding the upper part of thecapillary 8 can be suppressed. By this step, the contact between themetal ball 3b and bonding pad 2 at the joint portion 6a is secured, andre-contamination of the surface of the joint nucleus 6a by oxidation andthe like can be suppressed.

Next, at the time of applying supersonic wave vibration, the thirdbonding load is applied in the range of about 10 to 20 gf for a durationof about 10 ms, so as to make energy of supersonic wave vibration workin a concentrated manner in the region of not more than several μmcentering on each joint nucleus 6a, whereby it is possible to suppressextra vibration at the contact interface between the metal ball 3b andbonding pad 2 and to grow uniformly plural island-shaped joints 6b ofapproximately elliptical shape centering on the joint nucleus 6a. FIGS.3 to 6 show conditions of the joints in cases of the application of thefirst to third bonding load amounts at the respective joint temperaturesof 210, 230, 250, and 280° C. The size of the island-shaped joint 6b tobe formed is variable according to the joining temperature. In thetemperature range of the present experiment, there is a tendency thatthe lower the temperature is, the individual sizes of the island-shapedjoints 6b become smaller. Further, even at 210° C., the island-shapedjoint 6b is sufficiently formed.

FIG. 7 and FIG. 8 show the cases where amplitudes of the supersonic wavevibration were made smaller by 40% at the joining temperatures of 210°C. and 280° C., with other conditions kept at the same levels as thesamples of FIGS. 3 to 6. When the two cases are compared, it is seenthat the cases having the larger amplitude of the supersonic wavevibration (FIGS. 3 to 6) show formation of the island-shaped joint 6b inthe large size. Further, FIGS. 9 to 13 show the cases where time ofapplication of the third bonding load was changed to 3, 5, 10, 30, and60 ms at 210° C., showing that the longer the application time is, thelarger the island-shaped joint 6b grows. As shown in FIGS. 3 to 13, theindividual size of the island-shaped joint 6b can also be controlled bythe amplitude of supersonic wave vibration and time of application ofthe third bonding load.

FIG. 14 is a view to show the condition of the joint in the case wherethe first bonding load and the second bonding load were lowered to 100gf and 30 gf, respectively, and the third bonding load was applied inthe range of about 10 to 20 gf. Even in such a case, the island-shapedjoint 6b was formed. However, when, as shown in FIG. 15, the firstbonding load was set back to 120 gf and the second bonding load to 40 gfand the third bonding load was raised to 30 gf, it became impossible togive sufficient supersonic wave vibration energy to the neighborhood ofthe joint nucleus 6a to provide insufficient joint. Further, FIGS. 16and 17 are views to show conditions where the samples of jointtemperatures at 210° C. and 280° C. as shown in FIG. 3 were preserved at150° C. for 15 hours, and the island-shaped joint 6b could be observedeven after the preservation at a high temperature.

Further, in the final stage, the fourth bonding load is applied at ahigh amount of about 25 to 40 gf for the time of 3 to 5 ms so as to makeenergy of supersonic wave vibration work in a concentrated manner on aperipheral part of the joint of the metal ball 3b, whereby plasticdeformations of the metal ball 3b and bonding pad 2 are caused at theperipheral part of the metal ball 3b, and as shown in FIGS. 16 and 19,the band-like joint 7 can be formed in a manner to surround the whole ofplural island-shaped joints 6b.

In the above explanation, the first to fourth bonding load amounts shownin the present Embodiment are the amounts applicable to a metal ball 3bof about 55 μm in diameter formed at a tip of a metal wire 3a of 30 μmin diameter, and in the case where the size of the metal ball 3b isdifferent, the bonding load amount is required to be changed accordingto the size. That is to say, by making the amount obtained by dividingthe bonding load amount by the sectional area prior to deformation ofthe metal ball 3b to 40-50 mgf/μm² at the time of the first bondingload, 10-20 mgf/μm² at the time of the second bonding load, 4-10 mgf/μm²at the time of the third bonding load, and 10-20 mgf/μm² at the time ofthe fourth bonding load, there is obtained a joint comprising pluralisland-shaped joints 6b and a band-like joint 7 surrounding the whole ofthe island-shaped joints 6b in the same manner as this embodiment, thusmaking it possible to carry out highly reliable wire bonding. In suchcase, the time of applying the first bonding load is not more than 3 ms,the time of applying the third bonding load is 5-15 ms, and the time ofapplying the fourth bonding load is 1-5 ms.

In this embodiment, there is employed a die bond material comprisingpolyimide and epoxy resin as a main material, using phenol as a curingagent, and filled with silver powder. However, the components of theresin materials are not to be limited.

As described above, according to the wire bonding method of the presentEmbodiment, the joint nucleus 6a is sufficiently formed at the jointbetween the metal ball 3b and bonding pad 2; the deformation amount ofthe metal ball 3b can be properly set; it is possible to improveremarkably the reliability of fine wire bonding joints as represented byplastic package with fine pad pitch and multi-pin; and it is possible toprovide the high quality semiconductor device at a low price.

Embodiment 2

Hereinafter, description is given on the wire bonding method of thesecond embodiment of the present invention. Since the constitution ofthe semiconductor device to be used in this Embodiment is the same asthat of Embodiment 1, FIGS. 1 and 2 are quoted in making the followingexplanation. A semiconductor element 1 is connected to a lead frame 4comprising mainly Fe and Ni by using a die bond material 5 comprisingsolder as a main component. The following explanation is given on thecase where a metal wire 3a comprising mainly Au of 30 μm in diameter andhaving a metal ball 3b of about 55 μm in diameter, made by melting andsolidifying the tip of the metal wire 3a is joined to a bonding pad 2 of80 μm in one side comprising mainly Al and formed on the semiconductorelement 1, by using the load and supersonic wave energy of about 60 kHzsupplied through a capillary 8 and heat supplied from the lower face ofthe semiconductor element 1.

As in this Embodiment, in the case where the die bond material 5 is asolder, such material has a larger Young's modulus than the resin byabout one figure, so that it has high transmission efficiency ofsupersonic wave vibration. Because of this, the band-like joint 7 (referto FIG. 19) by the deformation of the metal ball 3b is more easilyformed by less supersonic wave vibration energy than the case of resindie bond material, even under the same temperature conditions.Accordingly, by suppressing the amplitude of supersonic wave vibrationat the time of application of the third bonding load to about 60% of thecase of the lead frame 4 comprising mainly Cu shown in Embodiment 1, itis possible to form the island-shaped joint 6b (refer to FIG. 19) overthe whole joint surface, as shown in FIGS. 20 and 21. In thisEmbodiment, at the joint temperatures of 210° C. (FIG. 20) and 280° C.(FIG. 21), the size of respective island-shaped joints 6b becomes smallat a low temperature. Subsequently, by applying the fourth bonding loadin the same manner as in the case of Embodiment 1, the band-like joint 7can be formed in a manner to surround the whole of plural island-likejoints 6b. Further, in the same manner as in Embodiment 1, even afterthe preservation at a high temperature, the form of the joint ispreserved.

In the above Embodiments 1 and 2, each of the first to fourth bondingload amounts can be further finely divided into the optional wave shapeswithin the range described in the explanation of the bonding loadamount. Further, while the frequency of the supersonic wave vibration isshown as about 60 kHz, the frequency might be about several hundred kHz.It is also possible to change the frequency and amplitude in one jointforming period. While there is taken as an example the case where themetal wire 3a comprising Au as the main component is to be joined to thebonding pad 2 comprising Al as the main component, metals such as Au,Ag, Cu, Al and Pt and their alloys and compounds might be used as amaterial for the metal wire 3a and bonding pad 2. Further, the wirediameter of the metal wire and the size of the bonding pad are notlimited. In the above Embodiments 1 and 2, description is made about abonding technique for joining the metal ball 3b with the bonding pad 2after forming a metal ball 3b by melting and solidifying the tip of themetal wire 3a. However, the present invention is also applicable to awedge bonding technique of directly joining the metal wire 3a with thebonding pad 2. Moreover, it is applicable to the joining at a roomtemperature.

Embodiment 3

Hereinafter, description is given on the wire bonding method of thethird embodiment of the present invention. This embodiment is to controlthe ratio of a band-like joint 7 to plural elliptical island-shapedjoints 6b formed at a joint interface between a metal ball 3b and abonding pad 2 according to the joining property of the bonding pad 2, asshown in FIGS. 1 and 2 and FIG. 22. The control of the ratio can berealized by adjusting the third and the fourth bonding loads and thetime of application of supersonic wave vibration. For example, in thecase where the bonding pad is thin, by increasing the ratio of the timeof application of the third bonding load as shown in FIG. 22(a), joiningcan be achieved without causing the exclusion of the bonding pad 2 by alarge plastic deformation amount. In its cross section, the ratio of theisland-shaped joint 6b is higher, and the ratio of the band-like joint 7is lower. To a bonding pad 2 having a low joining property, when theratio of the fourth bonding load application time is increased as shownin FIG. 22(b), joining can be achieved by utilizing the large plasticdeformation amount. In its cross-section, the ratio of the island-shapedjoint 6b is lower, and the ratio of the band-like joint 7 is higher.

Embodiment 4

FIG. 23 is a view to illustrate the constitution of the wire bondingapparatus of Embodiment 4 of the present invention. The presentapparatus is an apparatus for joining a metal wire with a bonding paddisposed on the semiconductor element by using a load and supersonicwave vibration. It is constituted by a stage (not illustrated) forplacing a semiconductor device including the semiconductor element and alead frame, a bonding head 11 for positioning the metal wire on thebonding pad of the semiconductor element with holding the metal wire, aload profile monitoring mechanism 12 for monitoring load profile of thebonding head 11, a conversion function or conversion table 13 to showcorrelation between the load profile and the strength and deformationamount of the wire bond joint, a load control mechanism 14 and asupersonic wave amplitude control mechanism 15 for calculating thesubsequent load amount and amplitude of the supersonic wave vibration onreceipt of the result from the monitor mechanism 12 to control thebonding head.

Next, description is given on the operation of the wire bondingapparatus according to this embodiment. This apparatus has, as shown inFIG. 1, a function to judge the forming amount of joint nucleus 6a(refer to FIG. 19) in the stage of elevating the first bonding load in asharp gradient after the metal ball 3b came into contact with thebonding pad 2. With respect to the judging method, a load profile ismeasured by a load gauge installed under the bonding stage, and based onthe load profile, reference is made to a conversion function or aconversion table 13 showing the correlation between the previouslymemorized load profile and the deformation amount and joint strength,and calculation is made to give the judgment results. These referenceand judgment results are transmitted to a load control mechanism 14 anda supersonic wave vibration control mechanism 15 to effect control ofthe bonding head. At such time, in order to elevate the precision ofjudgment, a mechanism to monitor a diameter of the metal ball 3b can beused simultaneously. For monitoring the diameter of the metal ball 3b,there might be employed, for example, monitor of electric power consumedat the time of the formation of the metal ball 3b.

When installation of a load gauge is difficult, the result of monitoringdeformation rate of the metal ball 3b can be utilized. As a procedurefor monitoring the deformation rate, a laser displacement gauge capableof measuring the displacement of the capillary without contact might beemployed. In the case where the mechanism for displacing the capillaryhas a displacement measuring mechanism such as an encoder, such meansmight be used to read the displacement rate. From such displacementinformation and the conversion function or conversion table representingcorrelation between the previously memorized displacement amount and thedisplacement amount and joint strength, subsequent pertinent third andfourth bonding load amounts and supersonic wave amplitude can bedetermined.

As described above, according to the wire bonding method and wirebonding apparatus of the present invention, by applying the first tofourth bonding load amounts for an optional duration, the joint nucleuscan be sufficiently formed at the joint portion between the metal wireand bonding pad, and deformation amount of the metal wire can beappropriately set, so that it is possible to improve the reliability ofthe fine wire bonding joint to a remarkable degree, and to provide ahigh quality semiconductor device at a low cost.

What we claim is:
 1. A wire bonding apparatus for joining a metal wirewith a bonding pad disposed on a semiconductor element, comprising:astage configured to place a semiconductor device including thesemiconductor element at a wire joining location; a bonding headconfigured to position the metal wire on the bonding pad while holdingthe metal wire and applying a load amount controlled by a load controlmechanism and supersonic wave vibration having a supersonic waveamplitude controlled by a supersonic wave amplitude control mechanism toform a joint; a mechanism configured to monitor change with time of theload amount applied by the bonding head; and a conversion function orconversion table element configured to provide a correlation between thechange with time of the load amount and the strength and deformation ofthe joint; wherein the load control mechanism is configured to calculateand implement a change in the load amount being applied by the bondinghead based upon the correlation and the supersonic wave amplitudecontrol mechanism is configured to calculate and implement a chance inthe amplitude of the supersonic wave vibration being applied by thebonding head based upon the correlation.